PROJECT SUMMARY/ABSTRACT The recent identification of novel genetic risk factors for Alzheimer's disease (AD) may help define new pathogenic mechanisms and therapeutic targets. BIN1 is the second-leading genetic risk factor for AD, after only APOE, and strong genetic, epigenetic, and neuropathological evidence now supports its association with AD. However, it remains unclear how BIN1 contributes to AD. Accumulating evidence points to reduction of the neuronal isoforms of BIN1 in AD, but the effect of losing these BIN1 isoforms on neuronal function and AD pathogenesis is a major knowledge gap. We have developed several tools to address the role of BIN1 in the brain, including BIN1 conditional knockout mice to reduce BIN1 selectively throughout the brain or in excitatory or inhibitory neurons, methods of expressing different BIN1 constructs in neurons either in culture or in vivo in specific neuronal populations, and primary neuron assays of the effects of BIN1 on neuronal activity. Our preliminary studies using these tools, described in this application, provide evidence for bidirectional regulation of neuronal activity by BIN1 and an important role in interneurons, which is particularly interesting in light of multiple data streams supporting a role for neuronal hyperexcitability in early stages of AD. The overarching hypothesis we will address in this proposal is that loss of neuronal BIN1 isoforms reduces neuronal activity, particularly in parvalbumin interneurons, leading to increased network hyperexcitability and impairment of gamma oscillations. This project will examine several aspects of this hypothesis. We will determine the effects of BIN1 loss in excitatory vs. inhibitory neurons on neuronal activity and susceptibility to network hyperexcitability, measuring the effects on BIN1 loss at the cellular level by patch-clamp electrophysiology and at the network level using EEG. We will determine mechanisms by which BIN1 regulates neuronal activity through structure-function analysis and manipulating BIN1 specifically in parvalbumin interneurons, measuring gamma oscillations and cognitive function. We will determine if lower BIN1 levels exacerbate pathogenesis in mouse models of AD, assaying the effects of both increasing and decreasing BIN1 in interneurons in mouse models of AD to directly test the hypothesis that loss of interneuron BIN1 contributes to A?-induced dysfunction. These studies will illuminate mechanism by which a leading genetic risk factor contributes to AD, identify specific neuronal populations involved, and determine the effects of these changes on brain networks contributing to cognition.